1. Field of the Invention
The present invention relates to a coal boiler and to a coal boiler combustion method.
2. Description of the Related Art
Boilers burn fuel to generate heat and generate steam through the use of the generated heat. Further, the boilers use the generated steam to drive a steam turbine and generate electrical power. However, boilers generating an electrical power of 500 MW or more have a 50 m or taller furnace and require a long construction period. An inverted boiler described, for instance, in JP-A-2003-314805 (claims and FIG. 1) and a transverse boiler described, for instance, in Japanese Patent No. 3652988 were invented to solve the above-mentioned problem. When such an inverted boiler or transverse boiler was used, the flow of a combustion gas was directed downward or sideways, respectively.
As regards a small-size boiler, a three-pass boiler is disclosed in a nonpatent document entitled “Steam, Its Generation and Use” (Babcock & Wilcox, 39th Edition, page 13-2). This three-pass boiler operates so that a combustion gas ejected from a burner sequentially flows upward, downward, and upward, and is discharged to the outside.
Meanwhile, if unburned carbon and NOx discharged from a boiler are to be reduced, it is important that the combustion time be increased. Such being the case, it was necessary, as described in JP-A-2002-81610, to increase the height of a furnace of a two-pass boiler in which the combustion gas ejected from a burner sequentially flows upward and downward and is discharged to the outside.